Image credit: ATLAS-PHO-Event-2015-008-1.
Following the restart of CERN’s flagship accelerator in early April, commissioning the LHC with beam is progressing well. In the early hours of 10 April, the operations team successfully circulated a beam at 6.5 TeV for the first time – a new world record – but this was only one of many steps to be taken before the accelerator delivers collisions at this beam energy.
The operators reached another important milestone on 21 April, when they succeeded in circulating a nominal-intensity bunch. The first commissioning steps in particular take place with low-intensity (probe) beams – single bunches of 5 × 109 protons. The nominal intensity, in contrast, is a little over 1 x 1011 protons per bunch, and when the LHC is in full operation later this year, some 2800 bunches will circulate in each beam.
Image credit: LHC/CERN.
To handle the higher number of protons per bunch and the higher number of bunches safely, a number of key systems have to be fully operational and set up with beam. These include the beam-dump system, the beam-interlock system and the collimation system. The latter involves around 100 individual pairs of jaws, each of which has to be positioned with respect to the beam during all of the phases of the machine cycle. Confirmation that everything is as it should be is made by deliberately provoking beam losses and checking that the collimators catch the losses as they are supposed to.
On 2 May, this set-up procedure allowed a nominal-intensity bunch in each beam to be taken to 6.5 TeV. Four days later, collisions were produced at the injection energy of 450 GeV, enabling the experiment teams to record events and check alignment and synchronization of the detectors. One of the important steps in reaching this stage is to commission the “squeeze” – the final phase in the LHC cycle of injection, ramp and squeeze. During this phase, the strengths of the magnetic fields either side of a given experiment are adjusted to reduce the beam size at the corresponding interaction point.
• To find out more, see the LHC reports in CERN Bulletin: bulletin.cern.ch.
